Form Ring Structures Research Articles

Rad52 forms ring structures and co-operates with RPA in single-strand DNA annealing.

The RAD52 epistasis group in Saccharomyces cerevisiae is involved in various types of homologous recombination including recombinational double-strand break (DSB) repair and meiotic recombination. A RecA homologue, Rad51, plays a pivotal role in homology search and strand exchange. Genetic analysis has shown that among members of its epistasis group, RAD52 alone is required for recombination between direct repeats yielding deletions. Very little has been discovered about the biochemical roles and structure of the Rad52 protein. Purified Rad52 protein binds to both single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). Electron microscope observations revealed that Rad52 molecules form multimeric rings. An increase in the intensity of fluorescence when Rad52 is bound to epsilonDNA showed an alteration of the structure of ssDNA. RPA was binding to Rad52 and enhanced the annealing of complementary ssDNA molecules. This enhancement was not observed in Escherichia coli SSB protein or T4 phage gp32 protein. Rad52 forms a ring-like structure and binds to ssDNA. Its structure and DNA binding properties are different from those of Rad51. The interaction of Rad52 with RPA plays an important role in the enhancement of annealing of complementary ssDNAs. We therefore propose that Rad52 mediates the RAD51-independent recombination through an ssDNA annealing, assisted by RPA.

Monte‐carlo modelling of the formation, structure and properties of polymer networks

AbstractA new Monte Carlo (MC) algorithm is reported which enables RAfa +RBfb polymerisations to be simulated. It is based on the random reaction of pairs of groups for intermolecular reaction, with intramolecular reaction weighted according to Gaussian statistics and the size of ring structure being formed. The algorithm counts all the ring structures formed and enables polymerisations to be simulated efficiently up to complete reaction. Gel point and modulus results from trifunctional and tetrafunctional polyurethane‐forming polymerisations are modelled, with particular emphasis on the effects on modulus of network defects at complete reaction. More in keeping with experiment, higher reductions in modulus at complete reaction are predicted using the the present algorithm than using Biosym MC software. However, it is shown that quantitative agreement with experiment cannot be obtained unless relationships defining the entropies of various ring structures are elucidated and more accurate modelling of the chain statistics governing the intramolecular reaction forming ring structures of various sizes is used.

Mapping the site of block by tetrodotoxin and saxitoxin of sodium channel II

The SS2 and adjacent regions of the 4 internal repeats of sodium channel II were subjected to single mutations involving, mainly, charged amino acid residues. These sodium channel mutants, expressed in Xenopus oocytes by microinjection of cDNA-derived mRNAs, were tested for sensitivity to tetrodotoxin and saxitoxin and for single-channel conductance. The results obtained show that mutations involving 2 clusters of predominantly negatively charged residues, located at equivalent positions in the SS2 segment of the 4 repeats, strongly reduce toxin sensitivity, whereas mutations of adjacent residues exert much smaller or no effects. This suggests that the 2 clusters of residues, probably forming ring structures, take part in the extracellular mouth and/or the pore wall of the sodium channel. This view is further supported by our finding that all mutations reducing net negative charge in these amino acid clusters cause a marked decrease in single-channel conductance.

Gravity fields of the terrestrial planets: Long‐wavelength anomalies and tectonics

We present a review of the long‐wavelength gravity fields of the terrestrial planets, earth, moon, Mars, and Venus with particular emphasis placed on the interrelationship between gravity anomalies and tectonic processes. After first summarizing appropriate statistical formulas, we discuss the relevant continuum mechanical solutions for elastic, viscoelastic, and convecting media in terms of the relationship to the gravity field both for predicting gravity anomalies and for the use of gravity as a constraining boundary condition. Gravity data can provide a strong constraint for the first two classes of solutions, but a significant ambiguity exists in convection interpretations because of the competing effects from flow and boundary deformation. The question of finite strength of crustal and mantle materials plays a paramount role in gravity studies because of the possibility of model discrimination based on stress states. A variety of evidence is reviewed, and it is concluded that the finite strength of the terrestrial planets is unlikely to exceed a kilobar when subject to loads over long geologic periods. Isostasy plays a dominant role in any discussion of the lithospheric contribution to planetary gravity fields. Since it tends to minimize stresses, it is a basic state to be expected, and most useful discussions of the gravity fields are centered on departures from the isostatic state, either due to dynamic forces or to finite strength. For the earth the long‐wavelength anomalies arise largely from beneath the lithosphere and are related to mantle flow. Apart from the obvious correlation of gravity to the plate boundaries, the specific relationships between the gravity field and relevant flow parameters are complex, and there are at present no systematic relationships established between geophysical observables in the intraplate regions. On the moon, most of the contributions to the gravity field arise from the lithosphere and are supported by the finite strength of that layer. The mascons are due to a combination of several kilometer thick basalt layers and crustal collapse at the time of impact, forming ring structures and mantle uplift. Mars appears to be intermediate between the earth and moon and is dominated by the Tharsis anomaly, with the possibility that gravity anomalies arise from both lithospheric and sublithospheric regions. If the Tharsis Province is supported entirely by the lithosphere, then stress levels are of the order of 1 kbar, even with isostasy, which may imply partial dynamic support. Preliminary analysis of Venus gravity data suggests a power spectrum grossly similar to that of the earth, but systematic discrepancies among some harmonics may indicate differences in the nature of interior processes for the two planets. In particular, there appears to be a higher correlation of long‐wavelength gravity with topography for Venus.